BUG/MINOR: httpclient: fix uninitialized sl variable

Reported by coverity in ticket #1355

  CID 1461505:  Memory - illegal accesses  (UNINIT)
  Using uninitialized value "sl".

Fix the problem by initializing sl to NULL.
diff --git a/src/http_client.c b/src/http_client.c
index 6142c48..7e950f9 100644
--- a/src/http_client.c
+++ b/src/http_client.c
@@ -437,7 +437,7 @@
 	struct channel *res = &s->res;
 	struct htx_blk *blk = NULL;
 	struct htx *htx;
-	struct htx_sl *sl;
+	struct htx_sl *sl = NULL;
 	int32_t pos;
 	uint32_t hdr_num;